1. Technical Field
The present invention relates to a radiological image detection apparatus used in the medical X-ray imaging system etc., and a method of manufacturing the same.
2. Related Art
In recent years, DR (Digital Radiography) using the X-ray image detection apparatus such as FPD (Flat Panel Detector) that converts an X-ray image into digital data, or the like is already put to practical use. In contrast to the former CR (Computed Radiography) system using the imaging plate formed of the stimulative phosphor (accumulative phosphor), this X-ray image detection apparatus has such a merit that the picked-up image can be checked on the spot, and thus its spread is proceeding apace.
Various systems have been proposed for an X-ray image detection apparatus. As one of them, the indirect conversion system, which converts the X rays into the visible lights once by the scintillator such as CsI:Tl, GOS (Gd2O2S:Tb), or the like, and then converts the visible lights into the electric charges by the semiconductor layers and stores such electric charges, has been known (see Patent Document 1 (JP-A-2007-163467), Patent Document 2 (JP-A-2008-51793) and Patent Document 3 (JP-A-2011-17683), for example).
In the X-ray image detection apparatus, in many cases it is preferable that the X-ray exposure should be set low when this detecting device is used for the X-ray radiography of a living body, for example. Therefore, the scintillator whose sensitivity to the X-rays is high and whose amount of luminescence is large is demanded. In Patent Document 1, an amount of luminescence is enhanced by providing the scintillator on both sides of the photodetector respectively to put it between them.
Also, in Patent Document 2, an amount of luminescence is enhanced by adding the activator to the base material of the fluorescent material. In Patent Document 2, it is set forth that, in the X-ray image detection apparatus which includes the photodetector and the scintillator and in which the X-rays are incident on the scintillator from the opposite side to the photodetector, the activator density in the region of the scintillator on the X-ray incident side should be enhanced.
Also, in Patent Document 3, an amount of luminescence is enhanced by setting the region of the scintillator, which is located in vicinity to the photodetector, as the main luminescence region S in the situation that the scintillator is irradiated with the X-rays from the photodetector side.
Here, it may be considered that an activator density on the X-ray incident side should be increased, as set forth in Patent Document 2, and also the photodetector side should be set as the main luminescence region, as set forth in Patent Document 3. In this manner, when an activator density is enhanced in vicinity to the photodetector on the X-ray incident side, the effect of increasing amount of luminescence and improving MTF (Modulation Transfer Function) can be achieved to a certain extent. However, when such main luminescence region of the scintillator is examined in detail, the following problems still remain. That is, an increase of an activator density poses clearly the technical problems mentioned hereunder.
The crystallinity of the part of the main luminescence region, which is located in vicinity of the photodetector, is disordered due to the increase of the activator density, and accordingly the degradation of MTF is caused. In particular, when an activator density is enhanced in the initial phase of the vapor deposition of the scintillator, such enhancement has a tremendous adverse influence on the crystal growth of the scintillator, and the crystallinity is disordered. Therefore, the lights are diffused between the columnar crystals, and thus degradation of MTF is caused.
Also, the absorption of lights in the scintillator is increased due to an increase of the activator density. Here, as shown in FIG. 14, such a case is considered that an activator density is enhanced in the situation that the part of a scintillator 91 located on the X-ray incident side is set as the main luminescence region S. As shown in FIG. 15A and FIG. 15B, in a part P2 that is positioned away from a photodetector 92 (FIG. 14) in the main luminescence region S, an amount of luminescence incident on the photodetector 92 is small, and a light emitting condition is spread, and thus blurriness of the image is caused (MTF is worsened). As a result, even though such a configuration is employed that, as shown in FIG. 14, the scintillator 91 is irradiated with the X rays from the photodetector 92 side, a further increase of an amount of luminescence and a further improvement in MTF cannot be expected unless such problems are solved.